This invention relates to a heater element consisting of a sintered body of a silicon nitride base ceramic and a resistance heating wire embedded in the ceramic body and a method of producing same.
In some of conventional ceramic heater elements a silicon nitride base ceramic is used as the material of the heater element body and a high melting point metal such as tungsten as the material of the resistance heating wire.
However, the silicon nitride base ceramic heater elements developed until now have a problem that in the sintered ceramic the grain boundary phase is a sort of glass phase which begins to soften as the temperature of the resistance heating wire in the ceramic body rises to about 1000.degree. C. by the flow of a current in the wire whereby the ceramic body lowers in mechanical strength, in particular in transverse strength. When a heater element using a silicon nitride base ceramic having such a grain boundary phase is operated with a DC voltage, as in the case of the heater element of a glow plug for a diesel engine, there arises another problem. That is, if the heater element is energized continuously or intermittently to reach a temperature above 1200.degree. C. the application of the DC voltage causes migration of ions in the grain boundary glass phase, and hence the structure of the ceramic body deteriorates with vacant holes created in a region near the positive terminal and microcracks in a region near the negative terminal. Consequently the heater element is liable to suffer from lowering of the strength of the ceramic body and/or breaking of the resistance heating wire. For these reasons the upper boundary of practical operational temperatures of the conventional ceramic heater elements is about 1150.degree. C.